A common solid state image sensor includes, as shown in FIG. 1, a construction having a plurality of photodiodes PD 10 arranged in the form of a matrix for generating image signal charges by converting light signals into electric signals, vertical charge coupled devices VCCD 12 each of which is arranged between each column of photodiodes PD 10 for transmitting the image signal charges generated in the photodiodes PD vertically, a horizontal charge coupled device HCCD 14 formed at one end of the vertical charge coupled devices for transmitting the image signal charges transmitted from the vertical charge coupled devices VCCD horizontally, and a sensing amplifier 16 formed at one end of the horizontal charge coupled device for sensing the transmitted image signal charges and transmitting them to outside in electric signals. Optical metal is formed on the foregoing construction. Also shown in FIG. 1 are the output 18, optical block area 20, image sensing area 22, and a unit cell 24.
A conventional solid state image sensor having the foregoing construction is explained in more detail hereinafter, referring to the attached drawings, wherein FIG. 2 is a layout of a unit cell of a conventional solid state image sensor; FIG. 3 is a section of a unit cell of a conventional solid state image sensor; and FIG. 4 shows an arrangement of the photodiodes and the vertical charge coupled devices of a conventional solid state image sensor.
First, as shown in FIG. 2, a unit cell of a conventional solid state image sensor includes a photodiode PD 28 and a vertical charge coupled device VCCD 30 having a fixed distance between them, a channel stop layer 32 formed around the photodiode PD except a part between the photodiode PD and the vertical charge coupled device VCCD, a first polygate 34 and a second polygate 36 for transmitting image signals from the photodiode PD to the vertical charge coupled device VCCD and in vertical direction within the vertical charge coupled device, and optical metal 38 for allowing only the photodiode PD to be irradiated. Herein, a transfer gate TG transmitting image signals from the photodiode to the vertical charge coupled device VCCD is the second polygate 36.
The sectional construction of the unit cell having the foregoing layout is as shown in FIG. 3. That is, the unit cell includes a p-type well 40 formed on an n-type semiconductor substrate 42, the photodiode PD and the vertical charge coupled device VCCD formed within the p-type well by forming an n-type impurity diffusion layer at a certain distance, a gate insulation film 44 deposited all over the surface, a polygate 46 formed over the vertical charge coupled device VCCD, an insulation film 48 deposited all over the surface, optical metal 50 (38) formed on all parts except the photodiode PD, and a color filter layer 52 and a microlens 54 formed over the photodiode. P-type material layer is shown by 55, 56 and N-type by 57, 58.
FIG. 4 shows a layout of photodiodes 60 and vertical charge coupled devices 62 of a conventional solid state image sensor, and FIG. 5 shows a layout of microlenses 70 of a conventional solid state image sensor, wherein each of the photodiodes has a rectangular shape in plan view and each of the microlenses over the photodiode PD has an oval shape. That is, since each of the photodiodes PD has a rectangular shape and the photodiodes are arranged in matrix, each of the microlenses formed over each of the photodiodes PD also has an oval shape and the microlenses are arranged in matrix, accordingly.
Operation of a conventional solid state image sensor having the foregoing construction is as follows.
Since the optical metal layer 38 shields light except from the photodiodes, light, focused by the microlenses, irradiates only onto the photodiodes. And the photodiodes PD generate image signal charges according to the quantity of light directed thereto. In this instance, the image signal charges generated in the photodiodes PD are transmitted to the vertical charge coupled devices VCCD through the first and the second polygates and by trigger voltage applied thereto, from each of the vertical charge coupled devices to the horizontal charge coupled device by clock signals V.phi.1 to V.phi.4, 74, 76, 78, 80 applied to the first and the second polygates, from the horizontal charge coupled device to output terminal by clock signals H.phi.1 to H.phi.2 and therefrom to outside in electric signals by the sensing amplifier at the output terminal.
However, the foregoing conventional solid state image sensor has the following problems.
First, the photodiodes PD have rectangular shapes and are arranged in a matrix, and the microlenses also are arranged in a matrix over each of the photodiodes, accordingly. Therefore, as shown in FIG. 5, even though the microlenses are arranged with a fixed space 84 between them, an empty space 86 formed by four adjacent microlenses occupies a relatively large space causing a drop in light receiving efficiency. Also, the shape of a photodiode is rectangular, and the shape of a microlens is oval. But its focusing shape is not rectangular. That is, as shown in FIG. 6, the shape of focusing of a microlens is like a peanut shell as has been published in the paper "1/3 inch 360K pixel IT-CCD Image Sensor, TO Institute, 1993, pages 192-200." Therefore, due to the difference of shape of light reception of a photodiode and the shape of focusing of a microlens, the efficiency of the cell layout drops.